Heat and Drought

Despite the rainfall that el Niño brings, California hasn’t yet recovered from its multi-year drought. Some have tried to deny any relationship between the drought and man-made climate change, but they usually miss the point entirely: that whether or not global warming increased the likelihood of the drought, it has indeed increased its severity.

New research by Woodhouse et al. drives home the point, not about the California drought but about the upper Colorado River basin. Water from the Colorado River is a big part of the supply for many western states (including California), and its availability has been on the decline. In some places, Lake Mead for example, the water level is troublingly low and continues to decline. This research, however, focused on the upper Colorado River basin by studying streamflow at Lees Ferry, and looking for possible relationships to precipitation, temperature, and soil moisture.

The best predictors they found were cold season precipitation (October through April) when the snowpack builds up, spring-summer temperature (March through July), and November soil moisture storage. All three are factors in the river’s streamflow, but it’s no surprise that the amount of precipitation is most important; that’s what determines how much water is in the snowpack, the main source for the upper Colorado. It’s not too hard to see its impact by plotting annual time series for both variables (flow during the water year, and Oct-Apr precipitation) on the same graph, using “normalized” data so they’re on roughly the same scale:

The relationship with temperature is also visually apparent, in a similar plot of normalized values:

The real test, however, is whether or not temperature is a significant predictor even after allowing for other factors. If we model the river flow as a function of both precipitation and soil moisture, then study what’s left over (the residuals), we can still see a relationship with temperature:

It turns out that yes, temperature is a factor in the Colorado river flow; higher temperature means reduced flow. This isn’t just due to the fact that both flow and temperature are trending (flow decreasing while temperature increases long-term); statistical significance remains (extremely strong) even if we allow for a simple time trend in our model.

What’s troubling is the size of the trend; river flow decreases about 10% for each degree Celsius temperature increase.

There is now a host of research establishing the relationship between temperature and drought severity, showing the effect of increased evaporation and changes to the timing and rapidity of snowmelt. This can have severe consequences beyond the problems associated with drought such as California has had to endure, things like lengthening of the wildfire season. Yet many will continue to deny any relationship between global warming and drought frequency or severity. Unfortunately for us, nature pays no mind to their eyes-wide-shut brand of skepticism, and when she shows us the consequences of our actions, we cannot help but see them.

18 responses to “Heat and Drought”

Every now and then I give my head a shake. Topics that would once have been limited to technical discussions between researchers that might have drawn polite, mild interest from outside observers are now matters of bitter public controversy.

The idea that warming could be accompanied by both more intense rainfall events AND more severe droughts isn’t hard to grasp. I’ve been to arid regions subject to seasonal heavy and the water is often gone within hours of falling… sucked into the dry ground or evaporated into hot dry air that follows the storms. And I’ve been in cool boreal swamps and taiga where every footstep squishes and squelches and the ground level humidity is 70 to 90% even if it hasn’t rained in weeks.

But these days American, Canadian, British or Australian researchers connecting drought to climate change not only have to contend with knowledgeable reviewers and colleagues who might pointedly dispute their data, hypotheses and analyses, but also an endless supply of highly motivated (and equally ignorant) idiots, with some of latter in positions of power or influence. Ugh. It’s a long slogging struggle.

Palmer, we are able to image carbon dioxide in the atmosphere by the reduction of infrared radiation reaching space. Measurements agree with airborne flask with a difference of less than 2 ppmv.

We have been able to demonstrate the absorption of infrared by carbon dioxide since the mid 1800s. We understand the absorption down to the quantum level in terms of the molecular bending mode of excitation.

The physics says and the evidence shows that it reduces the rate at thermal energy reaches space. The only way that the rate at which this energy will be brought back into equality with the rate at which energy enters the system is by heating the atmosphere and surface so that they radiate more infrared.

Ben – warmer temperatures with the same rainfall equals more severe drought. CO2 causes more heat to be retained in the climate system which causes the warmer temperatures. The ways a warmer world can affect the frequency, extent and duration of droughts gets more complex, but raised temperatures mean evaporation is increased and that impacts drought severity.

Your statement “river flow decreases about 10% for each degree Celsius temperature increase” seems to be wrong. Maybe 10% of normalized values, but since I assume the river doesn’t flow upstream (negative normalized values), there’s an offset being applied. Swings in river flow are rather large, with 1977 low point being 4.7MAF, and the 1983/84 high point being about 23MAF. I think one would need to understand the non-normalized contributions before a statement can be made about how much impact temperature has.

[Response: The mean flow during the study period (1906 through 2012) was 14.886 million acre-feet. The model estimates that each degree C reduces that by 1.473 million acre-feet. That’s 9.9% of the mean.]

Hydrologic drought is the running sum of “water out” minus “water in.” This conceptual definition underlies every drought severity index. It’s always driven by more than just changes in precipitation. Precip and snowfall are just descriptors of “water in.” Temperature is a primary control on “water out” through evaporative demand, which applies to changes in snowpack, runoff, reservoir storage, and especially warm-season evapotranspiration. The idea that CO2-induced global warming should increase drought is so straightforward that it’s almost a tautology:

Not “just as easily”. Numquam ponenda est pluralitas sine necessitate. We may or may not see more rainfall, but we’re already seeing evaporative demand increase with temperature, independent of changes in precipitation. That was likely the key factor, for example, in a regional die-off of Pinus edulis across northern New Mexico in the early 2000s.

Gerg: “Actually the effect of temperature on ‘evaporative demand’ is tricky.”

Agreed. As Fig. 1 of the linked paper shows, the period of reduced precipitation during 2000-2003 was not more prolonged or severe than that of 1953-1956, when many pinyon pines (Pinus edulis) also died; yet compared to the earlier die-off, the recent tree mortality was higher and more widespread. The key difference, according to the authors, is that it’s warmer now.

Like many species in the region, pinyon pine is adapted to sporadic drought. P. edulis trees respond to soil moisture deficit by closing their stomata to curtail water loss by transpiration. Photosynthesis effectively ceases as a result, requiring the trees to live on stored carbohydrates until moisture availability improves. Respiration, though, is a function of ambient temperature, so the trees deplete their carbohydrate reserves more rapidly as temperature increases, and can’t go as long without water.

The proximate cause of most of the tree mortality during 2002-2003 was infestation by a native bark beetle, pinyon ips (Ips confusus). P. edulis is also adapted to a typically low but constant rate of attack by these beetles. The trees defend themselves by engulfing beetles that penetrate their outer bark in copious pitch (and if you’ve ever gotten that stuff on your hands, you’ll know what the beetles are up against). With adequate moisture, healthy trees can manufacture enough pitch to resist historic rates of beetle attack indefinitely. Recent warmer winters allowed beetle populations to increase, however, forcing the trees to make more pitch from the same food reserves they needed to survive the drought. The trees in the study died when they could no longer make enough pitch to keep up with the beetles.

A follow-up study appears to confirm that tree death was more directly due to carbon starvation rather than hydraulic failure of the trees’ water-conducting xylem. Those trees arguably experienced warming as drought just the same, though. The proposed chain of causality is: